This invention relates generally to the production of extreme ultraviolet and soft x-rays for projection lithography.
The present state-of-the-art for Very Large Scale Integration (xe2x80x9cVLSIxe2x80x9d) involves chips with circuitry built to design rules of 0.25 xcexcm. Effort directed to further miniaturization takes the initial form of more fully utilizing the resolution capability of presently-used ultraviolet (xe2x80x9cUVxe2x80x9d) delineating radiation. xe2x80x9cDeep UVxe2x80x9d (wavelength range of xcex=0.3 xcexcm to 0.1 xcexcm), with techniques such as phase masking, off-axis illumination, and step-and-repeat may permit design rules (minimum feature or space dimension) of 0.18 xcexcm or slightly smaller.
To achieve still smaller design rules, a different form of delineating radiation is required to avoid wavelength-related resolution limits. One research path is to utilize electron or other charged-particle radiation. Use of electromagnetic radiation for this purpose will require x-ray wavelengths. Various x-ray radiation sources are under consideration. One source, the electron storage ring synchrotron, has been used for many years and is at an advanced stage of development. Synchrotrons are particularly promising sources of x-rays for lithography because they provide very stable and defined sources of x-rays, however, synchrotrons are massive and expensive to construct. They are cost effective only when serving several steppers.
Another source is the laser plasma source (LPS), which depends upon a high power, pulsed laser (e.g., a yttrium aluminum garnet (xe2x80x9cYAGxe2x80x9d) laser), or an excimer laser, delivering 500 to 1,000 watts of power to a 50 xcexcm to 250 xcexcm spot, thereby heating a source material to, for example, 250,000xc2x0 C., to emit x-ray radiation from the resulting plasma. LPS is compact, and may be dedicated to a single production line (so that malfunction does not close down the entire plant). The plasma is produced by a high-power, pulsed laser that is focused on a metal surface or in a gas jet. (See, Kubiak et al., U.S. Pat. No. 5,577,092 for a LPS design.)
Discharge plasma sources have been proposed for photolithography. Capillary discharge sources have the potential advantages that they can be simpler in design than both synchrotrons and LPS""s, and that they are far more cost effective. Klosner et al., xe2x80x9cIntense plasma discharge source at 13.5 nm for extreme-ultraviolet lithography,xe2x80x9d Opt. Lett. 22, 34 (1997), reported an intense lithium discharge plasma source created within a lithium hydride (LiH) capillary in which doubly ionized lithium is the radiating species. The source generated narrow-band EUV emission at 13.5 nm from the 2-1 transition in the hydrogen-like lithium ions. However, the source suffered from a short lifetime (approximately 25-50 shots) owing to breakage of the LiH capillary.
Another source is the pulsed capillary discharge source described in Silfvast, U.S. Pat. No. 5,499,282, which promised to be significantly less expensive and far more efficient than the laser plasma source. However, the discharge source also ejects debris that is eroded from the capillary bore and electrodes. An improved version of the capillary discharge source covering operating conditions for the pulsed capillary discharge lamp that purportedly mitigated against capillary bore erosion is described in Silfvast, U.S. Pat. No. 6,031,241.
Debris generation remains one of the most significant impediments to the successful development discharge sources in photolithography. The debris particles are ejected from the surfaces of the electrode (and also the capillary in the case of a capillary discharge source) caused by the short, intense pulses of electrical energy. These particles are generally small (less than one micron) and have very large velocities (greater than 100 m/s).
The present invention is based in part on the recognition that a gas curtain can be employed to protect multilayer optics from damage by debris that is generated by an EUV discharge source. It is expected that the gas curtain, e.g., a supersonic gas jet, will deflect sufficient amounts of debris generated by the EUV source without significantly reducing EUV transmission through the curtain. In addition, by efficiently removing the gas from the vacuum environment (e.g, chamber) in which the EUV source operates, the pressure therein can be maintained at an acceptable low level which prevents further EUV transmission attenuation.
In one embodiment, the invention is directed to a device that generates extreme ultraviolet and soft x-ray radiation that includes:
an EUV discharge source that produces a beam of radiation along a path and that generates debris; and
a gas curtain means for projecting a stream of gas over the path of radiation to deflect the debris into a direction that is different from that of the path of radiation.